1. Field of the Invention
The present invention relates to a device for detecting knocking in an internal combustion engine and, particularly, to a device for detecting knocking in an internal combustion engine capable of preventing noise caused by unstable combustion from being erroneously determined as an occurrence of knocking.
2. Prior Art
In an internal combustion engine using gasoline as a fuel, the gas mixture compressed by a piston is ignited by a spark plug and is burned to produce an output. That is, in normal combustion, a flame nucleus of the gas mixture is formed near the gap of the spark plug, and propagates over the whole combustion chamber.
The ignition timing of the spark plug has an intimate relationship with the output of the internal combustion engine. When the ignition timing is too late, the propagation speed of flame becomes slow. Therefore, the combustion becomes slow resulting in a decrease in the combustion efficiency and, hence, in a decrease in the output of the internal combustion engine.
When the ignition timing is early, on the other hand, the propagation of flame is quickened, whereby a maximum pressure of combustion increases and the output of the internal combustion engine increases. When the ignition timing is too early, however, there takes place knocking in which the mixture gas is self-ignited prior to the propagation of flame, often damaging the internal combustion engine.
That is, it is advantageous to operate the internal combustion engine in a region in which the ignition timing is set to just before the occurrence of knocking (MBT: minimum spark advance for best torque) from the standpoint of fuel efficiency and output. It is very important to reliably detect the occurrence of knocking.
A knock sensor which is a vibration sensor has heretofore been used for detecting knocking. However, a device has been studied which detects knocking by utilizing the phenomenon that ions are generated in the combustion chamber due to the combustion of the mixture gas and an ionic current flows.
FIG. 1 is a diagram schematically illustrating an ignition circuit for an internal combustion engine, wherein an end of a primary coil 111 of an ignition coil 11 is connected to the positive electrode of a battery 12. The other end is grounded via the collector and the emitter of a switching transistor 13 included in an igniter.
The base of the transistor 13 is connected to an ignition timing control unit 14, so that the transistor 13 is turned on when an ignition signal IGT is output from the ignition timing control unit 14.
An end of a secondary coil 112 of the ignition coil 11 is also connected to the positive electrode of the battery 12, and the other end is connected to a spark plug 8 through a reverse current-preventing diode 15, a distributor 16 and a high-tension cable 17.
An ionic current detecting unit 19 is connected to the output terminal of the distributer 16 in parallel with the spark plug 18.
The ionic current is supplied, through a protection diode 191, to a series circuit of a current-to-voltage conversion resistor 192 and a bias power source 193. A voltage generated at a point where the current-to-voltage conversion resistor 192 and the protection diode 191 are connected together, is supplied to an amplifying circuit 195 composed of an operational amplifier and a resistor through a capacitor 194 for removing DC components.
Therefore, a voltage signal proportional to the AC component of the ionic current is output at an output terminal 196 of the ionic current detecting unit 19.
FIGS. 2A to 2E are diagrams of voltage waveforms at each of the portions of the ignition circuit (FIG. 1), and shows, respectively an ignition signal IGT, a voltage on the grounding side of the primary coil (point-P), a voltage on the high-tension side of the secondary coil (point-S), and an input voltage to the amplifier circuit (point-I). All abscissa represent time.
When the ignition signal IGT assumes the "H" level at t.sub.1 the transistor 13 is turned on and the voltage at point P drops. Immediately after t.sub.1, a negative high-voltage pulse is generated at point S. However, the current is blocked by the reverse current-preventing diode 15 from flowing into the ionic current detecting unit 19.
When the ignition signal IGT is turned to the "L" level at t.sub.2 and the transistor 13 is cut off, a voltage at point P abruptly rises, and a positive high-tension pulse is generated at point S.
The positive high-voltage pulse is not blocked by the reverse current-preventing diode 15 and flows into the spark plug 18 to be discharged. It is prevented by the protection diode 191 from flowing into the ionic current detecting unit 19.
Furthermore, from t.sub.3 to t.sub.4 after the discharge of the spark plug 18, LC resonance is triggered by energy remaining in the ignition coil 11 due to parastic inductance and parastic capacitance of the high-tension cable 17 and the like.
The gas mixture in the cylinder is ignited by the discharge of the spark plug 18, ions are generated in the cylinder as the flame spreads, and an ionic current starts flowing. The ionic current increases with an increase in the pressure in the cylinder and decreases with a decrease in the pressure in the cylinder.
When knocking occurs in the internal combustion engine, knocking signals at a particular frequency (6 to 7 KHz) are superposed while the ionic current decreases after having reached its peak.
In order to detect the knocking using the ionic current, therefore, it is desired to detect only the knocking signals in a particular frequency band and reject other signals (e.g., an LC resonance one quency). For this purpose, therefore, it is desired to provide a knocking window which opens at t.sub.5 after no extra signal exists and closes at a suitable moment (e.g., ATDC 60.degree.) after the ionic current has decreased, and to detect the knocking based upon the output of the ionic current detecting unit 19 while the knocking window is opened.
The ionic current detecting unit 19, however, detects a very small ionic current. Therefore the amplifier circuit 195 must have a very large input impedance and a large gain, and inevitably picks up noise due to corona discharge of the spark plug 18.
In order to solve this problem, a method of detecting knocking in which the knocking frequency component only is picked up from the output of the ionic current detecting unit through a band-pass filter, and the knocking frequency component is integrated to reject the effect of noise has been already proposed (see Japanese Unexamined Patent Publication (Kokai) No. 6-159129).
FIG. 3 is a diagram illustrating the constitution of a conventional device for detecting knocking, and the output of the ionic current detecting unit 19 is supplied to a processing unit 34 through a band-pass filter (BPF) unit 32 and an integrating unit 33. The operation of the integrating unit 33 is controlled by a knocking window which is opened after a predetermined period depending upon the engine speed and the load, and is closed at a moment corresponding to about 50.degree. CA.
In the EGR (=exhaust gas recirculation) operation state in which the exhaust gases are recirculated to prevent NOx from being emitted by the internal combustion engine, however, the combustion in the combustion chamber of the internal combustion engine becomes unstable, and noise is superposed on the ionic current. The noise includes a frequency component close to that of knocking, and the output of the integrating unit increases so that the noise is inevitably and erroneously determined as the occurrence of knocking.
FIGS. 4A and 4B are diagrams explaining the problem, wherein FIG. 4A illustrates an ionic current when the internal combustion engine is operating at a maximum load (WOT=wide open throttle) and FIG. 4B illustrates an ionic current while the EGR is functioning. FIGS. 4A and 4B show the output of the ionic current detecting unit, the output of the band-pass filter, and the output of the integrating unit.
When the engine is operated at the maximum load, a first peak due to a generation occurrence flame and a second peak due to a rise in the pressure in the cylinder appear clearly on the output of the ionic current detecting unit after the LC resonance mask is opened. While the EGR is functioning, however, the combustion becomes unstable, the waveform becomes irregular, and the output of the integrating unit 33 is increased after the knocking window is opened, so that the misdetermination may be caused.
Though the EGR is used in a low-load region, unstable combustion is not always caused in the low-load region.
FIG. 5 is a diagram illustrating a relationship between the load and the output of the ionic current detecting unit, wherein the abscissa represents a pressure in the intake pipe (corresponds to the load) and the ordinate represents the output of the ionic current detecting unit. The length of an arrow upwardly extending from each point represents 3 .sigma. (three times as much as the variance of the noise amplitude).
When considering the WOT state as a reference point, an average ionic current and 3 .sigma. increase as the negative intake pressure becomes high, that is, as the load is decreased while the EGR is functioning. But while the EGR is functioning, however, the average ionic current and the 3 .sigma. decrease as the ignition angle is advanced. Even when the EGR is not operated, the average ionic current and 3 .sigma. decrease.
If a level for discriminating the knocking is determined based upon an average ionic current in the WOT state, therefore, an erroneous discrimination of knocking may be caused due to the unstable combustion depending upon the operating condition and, as a result, the ignition timing is controlled toward the delay side so that the output may decrease or the drivability may deteriorate.
The present invention was accomplished in view of the above-mentioned problems, and provides a device for detecting knocking in an internal combustion engine which is capable of preventing noise caused by unstable combustion from being erroneously discriminated as the occurrence of knocking.